Think of an airlock as it's supposed to be thought of, as a VENT a VALVE to release gas, NOT as a fermentation gauge. All that more than likely happened is gas that has been present ALL THE TIME in solution got released, either from it being trapped in the yeast at the bottom of the vessel, because of some physical shaking of the vessel or the room it is being stored in, or the change in barometric pressure putting pressure on the liquid in the vessel or a rise in ambient temp causing the co2 to expand beyond the layer that was in the headspace. Or opening the fermenter to take a gravity reading....
Or because they moved the fermenter.
Airlocks bubble for many reasons Barometric changes/Temp changes/ A truck rumbling by on the street/The cat brushing against it/Vacuum Cleaner....Bubbling of an airlock, especially if it's been idle for awhile, is usually a product of changes in the environment, rather than anything else.
That's what so many brewers into trouble, they keep thinking an airlock is a fermentation gauge, that any time it acts up or doesn't act up they think fermentation is or isn't occurring. If you think of it as it's REAL purpose, as a release valve to keep the beer off the ceiling, then you realize that you dealing with gas which is fluid in it's own right, it expands, contracts, and comes out of solutions form many reasons beyond JUST fermenting....
It's offgassing because you racked it to secondary and kicked up co2, that's NOT the same thing as fermenting.
